.TH std::unordered_set::find 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::unordered_set::find \- std::unordered_set::find

.SH Synopsis
   iterator find( const Key& key );             \fB(1)\fP \fI(since C++11)\fP
   const_iterator find( const Key& key ) const; \fB(2)\fP \fI(since C++11)\fP
   template< class K >                          \fB(3)\fP \fI(since C++20)\fP
   iterator find( const K& x );
   template< class K >                          \fB(4)\fP \fI(since C++20)\fP
   const_iterator find( const K& x ) const;

   1,2) Finds an element with key equivalent to key.
   3,4) Finds an element with key that compares equivalent to the value x. This
   overload participates in overload resolution only if Hash::is_transparent and
   KeyEqual::is_transparent are valid and each denotes a type. This assumes that such
   Hash is callable with both K and Key type, and that the KeyEqual is transparent,
   which, together, allows calling this function without constructing an instance of
   Key.

.SH Parameters

   key - key value of the element to search for
   x   - a value of any type that can be transparently compared with a key

.SH Return value

   An iterator to the requested element. If no such element is found, past-the-end (see
   end()) iterator is returned.

.SH Complexity

   Constant on average, worst case linear in the size of the container.

.SH Notes

           Feature-test macro          Value    Std                Feature
                                                      Heterogeneous comparison lookup
   __cpp_lib_generic_unordered_lookup 201811L (C++20) in unordered associative
                                                      containers; overloads (3,4)

.SH Example


// Run this code

 #include <cstddef>
 #include <functional>
 #include <iostream>
 #include <source_location>
 #include <string>
 #include <string_view>
 #include <unordered_set>

 using namespace std::literals;

 namespace logger { bool enabled{false}; }

 inline void who(const std::source_location sloc = std::source_location::current())
 {
     if (logger::enabled)
         std::cout << sloc.function_name() << '\\n';
 }

 struct string_hash // C++20's transparent hashing
 {
     using hash_type = std::hash<std::string_view>;
     using is_transparent = void;

     std::size_t operator()(const char* str) const
     {
         who();
         return hash_type{}(str);
     }
     std::size_t operator()(std::string_view str) const
     {
         who();
         return hash_type{}(str);
     }
     std::size_t operator()(std::string const& str) const
     {
         who();
         return hash_type{}(str);
     }
 };

 int main()
 {
     std::unordered_set<int> example{1, 2, -10};

     std::cout << "Simple comparison demo:\\n" << std::boolalpha;
     if (auto search = example.find(2); search != example.end())
         std::cout << "Found " << *search << '\\n';
     else
         std::cout << "Not found\\n";

     std::unordered_set<std::string, string_hash, std::equal_to<>> set{"one"s, "two"s};

     logger::enabled = true;
     std::cout << "Heterogeneous lookup for unordered containers (transparent hashing):\\n"
               << (set.find("one")   != set.end()) << '\\n'
               << (set.find("one"s)  != set.end()) << '\\n'
               << (set.find("one"sv) != set.end()) << '\\n';
 }

.SH Possible output:

 Simple comparison demo:
 Found 2
 Heterogeneous lookup for unordered containers (transparent hashing):
 std::size_t string_hash::operator()(const char*) const
 true
 std::size_t string_hash::operator()(const std::string&) const
 true
 std::size_t string_hash::operator()(std::string_view) const
 true

.SH See also

   count       returns the number of elements matching specific key
               \fI(public member function)\fP
   equal_range returns range of elements matching a specific key
               \fI(public member function)\fP
